1. Field of the Invention
The present invention relates to a magnetic levitating transportation apparatus including a vehicle capable of travelling along a rail.
2. Description of the Related Art
In recent semiconductor manufacturing industries, wafers are often automatically transported in wafer treatment processes so that the wafers are contaminated by as little dust as possible. A magnetic levitating transportation apparatus such as a linear motor car is suitable for an automatic transportation apparatus for wafers because there is no friction between the vehicle and the rail and thus dust is not generated, and it is possible to transport wafers quickly and reliably.
The magnetic levitating transportation apparatus includes a vehicle capable of being levitated relative to a rail installed along a predetermined path with a gap between the vehicle and the rail. The vehicle travels along the rail. The vehicle is driven along the rail by a linear motor and caused to magnetically levitate by magnet units which are arranged on the vehicle. Also, gap sensors are arranged on the vehicle for detecting the gap between the vehicle and the rail to control the current supplied to the magnet units to maintain the gap between the vehicle and the rail at a desired constant value.
Four magnet units, for example, are arranged on the vehicle. Each magnetic unit is of a composite structure comprising a permanent magnet and electro-magnets. A relatively large current is supplied to the coils of the electro-magnets at the initial stage of levitation of the vehicle so that the vehicle is quickly levitated by the sum of the magnetic forces of the permanent magnet and the electro-magnets, and the current supplied to the coils of the electro-magnets is gradually reduced as the vehicle levitates. The vehicle finally levitates with a constant gap between the vehicle and the rail. It is also possible to supply the current to the coils in the reverse direction, so that a difference between the magnetic forces of the permanent magnet and the electro-magnets acts on the vehicle and the vehicle so as to enlarge the gap between the vehicle and the rail.
It is desirable that the final gap between the vehicle and the rail be such that the current supplied to the electro-magnets of the magnet units is substantially zero and the vehicle is supported by the magnetic force of only the permanent magnets. That is, if the weight of the vehicle balances the magnetic force of the permanent magnets, it is possible to minimize the current supplied to the electro-magnets of the magnet units and thus to minimize power consumption from the battery mounted in the vehicle.
Four magnet units, for example, are arranged on the vehicle in a common plane. The current is controlled separately for each of the magnet units. The vehicle may not necessarily be parallel to the rail depending on the position of the vehicle and the position of the load on the vehicle. Therefore, the gap between one magnet unit and the rail when the current supplied to one magnet unit becomes substantially zero may differ from the gap between another magnet unit and the rail when the current supplied to the latter magnet unit becomes substantially zero. For example, the gap between one magnet unit and the rail when the current supplied to one magnet unit becomes substantially zero may be approximately 1.8 mm, while the gap between another magnet unit and the rail when the current supplied to the latter magnet unit becomes substantially zero may be approximately 1.9 mm.
In the case where four magnet units are arranged in a common plane, a problem arises that it is not possible to reduce the current supplied to all electro-magnets of the magnet units to substantially zero. For example, when the gaps of three magnet units are determined, the gap of the fourth magnet unit is automatically determined from the relationship of the plane. However, this determined gap of the fourth magnet may not necessarily correspond to the gap at which the current can be reduced to zero. Therefore, it is not possible to reduce the current supplied to the electro-magnets of the last magnet unit.